U.S. patent number 6,049,412 [Application Number 09/158,670] was granted by the patent office on 2000-04-11 for reflective faraday-based optical devices including an optical monitoring tap.
This patent grant is currently assigned to Lucent Technologies, Inc.. Invention is credited to Ernest Eisenhardt Bergmann, Terry William Cline, Stephen Kenneth Fairchild.
United States Patent |
6,049,412 |
Bergmann , et al. |
April 11, 2000 |
Reflective Faraday-based optical devices including an optical
monitoring tap
Abstract
A reflective Faraday-based optical system is formed to include a
partially transmissive optical reflector so that a portion of the
optical signal propagating through the system will not be reflected
but instead used as an input signal to a monitoring system. The
partially transmissive reflector is configured to transmit only a
relatively small portion of the optical signal (about 1-10%) so
that the performance of the system is not affected. The optical
monitoring arrangement may comprise one or more photodetectors,
optical fibers, or other optical components for capturing the
transmitted signal and converting into an electrical representation
that can be evaluated to monitor the power in the optical
signal.
Inventors: |
Bergmann; Ernest Eisenhardt
(Borough of Fountain Hill, Lehigh County, PA), Cline; Terry
William (Bethlehem Township, Northhampton County, PA),
Fairchild; Stephen Kenneth (Maxatawny Township, Berks County,
PA) |
Assignee: |
Lucent Technologies, Inc.
(Murray Hill, NJ)
|
Family
ID: |
22569173 |
Appl.
No.: |
09/158,670 |
Filed: |
September 22, 1998 |
Current U.S.
Class: |
359/301; 359/280;
359/283; 359/298 |
Current CPC
Class: |
G02F
1/09 (20130101); G02F 1/093 (20130101); G02F
2201/34 (20130101); G02F 2203/09 (20130101) |
Current International
Class: |
G02F
1/01 (20060101); G02F 1/09 (20060101); G02B
026/08 (); G02F 001/29 () |
Field of
Search: |
;359/280-283,298,301.302,304,284 ;356/218,285,358 ;372/31,106 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Epps; Georgia
Assistant Examiner: Spector; David N.
Claims
What is claimed is:
1. A reflective Faraday-based optical system for conveying an
optical signal from an input signal port to an output signal port,
said system comprising
a Faraday rotator disposed in the signal path between said input
port and said output port;
a partially transmissive reflector disposed in the optical signal
path beyond said Faraday rotator for re-directing an optical signal
exiting said Faraday rotator through said Faraday rotator in the
reverse direction, said reflector also for providing a portion of
said optical signal as a transmitted optical signal in a second
path distinct from said reflected signal; and
an optical monitoring arrangement disposed beyond said partially
transmissive reflector for capturing said transmitted optical
signal and generating an output signal representative of the power
of said optical signal, said monitoring arrangement comprising a
pair of photodiodes, a first photodiode for capturing an optical
signal propagating through said optical system in a forward
direction and a second photodiode for capturing an optical signal
propagating through said optical system in a reverse direction.
2. A reflective Faraday-based optical system for conveying an
optical signal from an input signal port to an output signal port
said system comprising
a Faraday rotator disposed in the signal path between said input
port and said output port;
a partially transmissive reflector disposed in the optical signal
path beyond said Faraday rotator for re-directing an optical signal
exiting said Faraday rotator through said Faraday rotator in the
reverse direction, said reflector also for providing a portion of
said optical signal as a transmitted optical signal in a second
path distinct from said reflected signal;
an optical monitoring arrangement disposed beyond said partially
transmissive reflector for capturing said transmitted optical
signal and generating an output signal representative of the power
of said optical signal; and
at least one optical processing component disposed in the signal
path between the partially transmissive reflector and the optical
monitoring arrangement for discriminating between optical signals
based upon predetermined optical properties.
3. An optical system as defined in claim 2 wherein the at least one
optical processing component is capable of discriminating between
signals based upon propagation direction.
4. An optical system as defined in claim 2 wherein the at least one
optical processing component is capable of discriminating between
optical signals based upon the polarization states of the optical
signals.
5. An optical system as defined in claim 2 wherein the at least one
optical processing component is capable of discriminating between
optical signals based upon optical wavelength.
6. An optical system as defined in claim 2 wherein the at least one
optical processing component comprises a focusing element; and
the optical monitoring system comprises a pair of optical
waveguides disposed in proximity to each image point of said
focusing element.
7. An optical system as defined in claim 6 wherein the optical
monitoring system further comprises
photodetecting means coupled to receive the output from the pair of
optical waveguides and convert the optical signals into electrical
equivalent; and
monitoring means for receiving the electrical output signals from
said photodetecting means and generating a monitoring signal
representative of the power of the optical signal.
8. An optical system as defined in claim 6 wherein the optical
monitoring system comprises
an input optical waveguide disposed in proximity to a first image
point of said focusing lens.
9. An optical system as defined in claim 8 wherein the optical
monitoring system further comprises
photodetecting means coupled to receive the output from the input
optical waveguide and convert the optical signals into electrical
equivalent; and
monitoring means for receiving the electrical output signals from
said photodetecting means and generating a monitoring signal
representative of the power of the optical signal.
10. An optical system as defined in claim 2 wherein the at least
one optical processing component comprises a filter for separating
multiple wavelengths present in an optical signal so as to allow
for only a predetermined set of wavelengths to be transmitted
through to the optical monitoring arrangement.
11. An optical system as defined in claim 2 wherein the optical
component comprises a polarizer for blocking a predetermined
polarization direction to pass into the optical monitoring
arrangement.
Description
BACKGROUND OF THE INVENTION
The present invention relates to reflective Faraday-based optical
devices and, more particularly, to such devices including a
partially transmissive reflector element and photodetection
arrangement to monitor the optical signals propagating through the
device.
DESCRIPTION OF THE PRIOR ART
There are many optical devices that require a "non-reciprocal"
rotation of the optical signal traversing the system--optical
isolators and circulators exemplify such devices. Faraday rotators,
such as garnet films, are often used to provide this non-reciprocal
rotation. In the interest of reducing the size of such systems,
"reflective" isolators and circulators have been developed that
essentially halve the number of required components by including a
reflector at the midpoint of a conventional arrangement and thus
reflect the optical signal back through the same components. U.S.
Pat. No. 5,191,467 issued to Kapany et al. on Mar. 2, 1993
discloses an exemplary prior art reflective optical isolator. A
circulator incorporating a reflective component is disclosed in
U.S. Pat. No. 5,471,340.
A separate interest exists in the capability to constantly monitor
the optical power within optical devices as a means to monitor and
maintain proper functioning of optical systems. There exist other
arrangements that utilize reflections within an optical system as
"tap" signals that are thereafter coupled to monitoring equipment
to analyze the operation of the system.
A need remains in the art for incorporation such an optical tap
arrangement in reflective Faraday-based optical systems, where the
reflections available in prior art systems are not as
prevalent.
SUMMARY OF THE INVENTION
The need remaining in the prior art is addressed by the present
invention, which relates to reflective Faraday-based optical
devices and, more particularly, to such devices including a
partially transmissive reflector element and photodetection
arrangement to monitor the optical signals propagating through the
device.
In accordance with the present invention, the reflective
element/mirror included in the prior art arrangements mentioned
above is replaced by a partially transmissive reflective element.
Such elements, for example, a multi-layer thin film stack of proper
design, allow for approximately 90% of the signal to be reflected
and about 10% to be transmitted. By reflecting a majority of the
signal, the system maintains sufficient optical signal strength to
operate properly, while a 10% transmissive factor is more than
sufficient to allow for the signal quality to be analyzed. In other
embodiments, a 99% reflectivity and 1% transmission may be
appropriate. In general, any appropriate split between reflected
and transmitted signal may be used and is considered to fall within
the scope of the preset invention.
In one embodiment, a single large-size photodetector may be used to
monitor optical signals traveling in either direction through the
optical device. Alternatively, separate monitors can be used to
detect signals traveling in the "forward" and "reverse" directions,
respectively.
Additional elements such as lenses, polarizers, filters, etc. may
be disposed in the signal path between the partially transmissive
reflector and the photodetector to further modify the optical tap
signal.
Other and further features of the present invention will become
apparent during the course of the following discussion and by
reference to the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
Referring now to the drawings, where like numerals represent like
components in several views:
FIG. 1 illustrates an exemplary reflective Faraday-based optical
isolator incorporating an optical tap arrangement of the present
invention including a single photodetecting device;
FIG. 2 illustrates an alternative optical isolator arrangement,
including an optical tap of the present invention including a pair
of photodetecting elements;
FIG. 3 illustrates an exemplary reflective Faraday-based optical
isolator and optical tap arrangement, including lensing disposed
between the partially transmissive reflector and a pair of
photodetecting devices;
FIG. 4 is an alternative embodiment of the arrangement of FIG. 3,
incorporating the lensing element into the partially transmissive
reflector;
FIG. 5 is an exemplary reflective Faraday-based optical isolator
and optical tap arrangement, including additional optical
components (such as a polarizer or filter) disposed in the signal
path between the partially transmissive reflector and the
photodetecting arrangement; and
FIG. 6 is an exemplary reflective Faraday-based optical circulator
including an optical tap arrangement of the present invention;
FIG. 7 is a side view of the arrangement of FIG. 6, illustrating
the positioning of the optical ports relative to the first walk-off
device; and
FIG. 8 illustrates an exemplary arrangement of the present
invention utilized as a Faraday reflector.
DETAILED DESCRIPTION
An exemplary optical system 10 incorporating the optical tap
arrangement of the present invention is illustrated in FIG. 1.
System 10 is defined as a reflective optical isolator and includes
an input signal path 12 and output signal path 14. As is well-known
in the art, an optical isolator functions to allow signals
propagating in the "forward" direction to travel unimpeded, while
blocking any signals traveling in the "reverse" direction from
being coupled into the input signal path. In the arrangement of
FIG. 1, therefore, optical signals (whose polarization allows them
to pass through polarizer 16) coupled into input path 12 will
propagate through the system and exit along output signal path 14,
while any signals injected into signal path 14 will be prevented
from entering signal path 12. As shown, system 10 further comprises
a linear polarizer 16 disposed along input signal path 12 and
oriented at a predetermined angle .theta., and an analyzer 18
oriented at .theta.+45.degree. and disposed along output signal
path 14. A Faraday rotator 20 is disposed in the signal path beyond
both devices 16 and 18. Since this is a "reflective" system, an
input signal will traverse Faraday rotator 20 twice; once in the
"forward" direction and once in the "reverse" direction. As such,
Faraday rotator 20 is devised to impart only a 22.5.degree.
rotation to the signal on each pass (thus resulting in a complete
45.degree. rotation after both passes). The arrangement of the
present invention therefore provides transmission in the forward
direction (i.e., rotation from .theta. to .theta.+45.degree.) and
isolation in the reverse direction (i.e., rotation from
.theta.+45.degree. to .theta.+90.degree.).
In accordance with the present invention, a partially transmissive
reflector 22 is disposed in the signal path beyond Faraday rotator
20. Partially transmissive reflector 22 includes a concave,
reflective front surface 24 that functions to properly re-direct
and refocus the reflected portion of the optical signal from the
"upper" portion of Faraday rotator 20 into the "lower" portion, as
illustrated in FIG. 1. Importantly, due to the partially
transmissive nature of reflector 22, a portion of the optical
signal will pass through reflector 22 and exit through its rear
surface 26 (in one embodiment, rear surface 26 may include an
anti-reflective coating). For the purposes of the present
invention, a partially transmissive device that reflects
approximately 90% of the signal and transmits approximately 10% is
appropriate (as mentioned above, other combinations are possible,
such as 99/1, 95/5, etc.). As shown, the transmitted portion of the
optical signal thereafter impinges a large area photodetector 28,
which converts the captured optical signal into an electrical
representation and provides the electrical output as a monitoring
signal to a monitoring device, such as meter 30. Due to the size of
photodetector 28 with respect to the optical signal paths,
alignment of detector 28 with respect to partially transmissive
reflector 22 is trivial.
As described above, an optical isolator functions to block any
un-intended signals propagating in the reverse direction through
the device from being injected into the input signal path (signal
path 12 for the arrangement of FIG. 1). A reverse-propagating
signal is illustrated by the dashed line in FIG. 1. Since this
signal will not yet be blocked as it passes through partially
transmissive reflector 22, a portion of this reverse-directed
signal will also impinge photodetector 28. For some cases, this is
of no concern. However, in certain circumstances it may be
desirable to be able to distinguish the monitoring of
forward-directed signals from reflections passing in the reverse
direction through the optical system.
FIG. 2 illustrates an alternative embodiment of the present
invention that utilizes a pair of large area photodetectors 32, 34
in place of the single large area photodetector 28. Optical system
36 of FIG. 2 includes the same optical components as discussed
above in associated with FIG. 1 and need not be described again. As
illustrated, however, by using a pair of photodetectors, it is
possible to separately monitor the power of an optical signal
propagating in the forward direction (i.e., from signal path 12)
and any return signal propagating in the reverse direction (i.e.,
from signal path 14). In particular, a transmitted portion of an
optical signal propagating in the forward direction will exit
partially transmissive reflector 22 and impinge a first photodiode
32, where photodiode 32 is properly aligned with respect to
reflector 22 to capture only the forward-directed signal.
Photodiode 32 functions to convert this signal into an electrical
equivalent which is thereafter provided as an input to a monitoring
arrangement 38. Any return optical signal passing through reflector
22 will be coupled into a second photodiode 34, as shown in FIG. 2
and converted into an electrical signal applied as an input to
monitoring arrangement 38.
In some optical systems it may be problematical to locate
photodetectors in such close proximity to the partially
transmissive reflector. FIG. 3 illustrates an optical system 40
that provides for a more "remote" monitoring arrangement. As shown,
the optical tap arrangement of system 40 includes a focusing means
42 (for example, a lens) disposed in the signal path beyond
partially transmissive reflector 22. A first optical fiber 44 is
positioned at a first image point beyond focusing means 42 and is
located so that the forward-directed signal will be focused into
the core region of fiber 44. Similarly, a second optical fiber 46
is positioned at another image point associated with focusing means
42 such that any reflected signal will be coupled into second fiber
46. Fibers 44 and 46 may then extend as far as necessary to a
"remotely located" monitoring arrangement 48, where monitoring
arrangement 48 may comprise photodetectors and an electrical signal
monitor, as described above. Fibers 44 and 46 may comprise either
multimode fiber or single mode fiber, where the larger core size
associated with multimode fiber will simplify the alignment of the
fibers with respect to focusing means 42. Indeed, elements 44 and
46 may comprise any suitable type of optical waveguide capable of
supporting the transmission of the optical signals to a remotely
located monitoring arrangement. For the purposes of the present
discussion, elements 44 and 46 will be referred to as "fibers", but
it is to be understood that any suitable type of waveguide may be
used.
Further, focusing means 42 could comprise any suitable arrangement,
including but not limited to, a curved lens or curved mirror.
Alternatively, focusing means 42 may be incorporated into the
partially transmissive reflector. FIG. 4 illustrates an optical
system 50 where individual components 22 and 42 of system 40 are
replaced by a single partially transmissive reflector 52. As shown
in FIG. 4, partially transmissive reflector 52 is formed to include
a reflective front surface 54 and a curved rear surface 56, where
the curvature of surface 56 is sufficient to provide the necessary
focusing into monitoring fibers 44,46.
Components other than focusing arrangements may be included in the
optical tap monitoring system of the present invention. FIG. 5
illustrates an exemplary system 60 that includes a generalized
optical component 62 disposed between partially transmissive
reflector 22 and a detector arrangement 64 (detector arrangement 64
may comprise any of the various arrangements described above, as
well as any other suitable optical detection arrangement). In one
embodiment, component 62 may comprise an optical filter. When used
in conjunction with an optical amplifier, an optical filter is
useful in separating the pump wavelength from the signal wavelength
so that the detector arrangement monitors only the signal power and
ignores the pump power (or vice versa, if desired). An optical
filter may also be used in a wavelength division multiplexed system
to distinguish among the various wavelengths comprising the system.
Alternatively, component 62 may comprise a polarizer so that when
used with an isolator or circulator, the detection arrangement is
capable of distinguishing (either partially or totally) "forward"
signals from "reflected" signals. Since the polarization of the
forward and backward signals are different, one direction may be
"rejected" by a suitable analyzer 62 (that may function to
partially attenuate the desired signal propagating in the remaining
direction). Combinations of focusing means with these components is
also possible and is considered to fall within the spirit and scope
of the present invention.
There are other Faraday-based reflective optical systems that may
include an optical tap arrangement of the present invention. FIG. 6
illustrates an exemplary optical partial circulator 70 including a
three-tap optical monitoring arrangement 72. The view illustrated
in FIG. 6 is a top view of partial circulator 70, in the x-z plane
of the device. A side view of partial circulator 70 is illustrated
in FIG. 7, this view being taken along the y-z plane. As shown in
the view of FIG. 6, partial circulator 70 includes a set of three
optical signal ports 74, 76 and 78, disposed as shown at a first
end of circulator 70. In general, a first signal injected into port
74 will propagate through the system and exit at port 76.
Similarly, a signal injected into port 76 will propagate through
the system and exit at port 78. The particular partial circulator
arrangement of FIGS. 6 and 7 includes a first walk-off device 80.
Device 80 is, for example, a birefringent crystal that functions to
separate the polarization components of signals exiting any of the
signal ports 74, 76 or 78 and, in the reverse direction, will
combine the separate polarizations for signals to be coupled into
ports 74, 76 or 78 (lensing elements may be disposed between the
input signal ports and device 80 to provide additional focusing).
Disposed beyond walk-off device 80 is a pair of half-wave plates
82,84, where the position of these plates is best shown in FIG. 7.
Referring to FIG. 7, the position of input signal ports 74,76 and
78 relative to half-wave plates 82,84 is evident. That is, the set
of ports are disposed below the center line C of partial circulator
70, the center line defined by the line of demarcation between
upper half-wave plate 82 and lower half-wave plate 84. In
particular, upper half-wave plate 82 is positioned with its "fast"
and "slow" axes off-parallel by 22.50 to the y and x axes,
respectively. In contrast, lower half-wave plate 84 is oriented at
an angle of 22.5.degree. in the opposite direction with respect to
the x axis. Therefore, the polarizations of the upper and lower
beams are effectively rotated by .+-.45.degree., respectively.
Since these two beams were originally polarized vertically and
horizontally, they now have parallel polarizations at 45.degree. to
the vertical. A first Faraday rotator 86 is disposed beyond the
pair of half-wave plates 82,84 and provides a 45.degree. clockwise
rotation to each signal component passing therethrough so that both
signal components are vertically polarized. The components exiting
first Faraday rotator 86 are then coupled into a second
birefringent device 88. The beam shift axis of device 88 is
oriented parallel to the X axis, allowing the components to pass
through unchanged. A second Faraday rotator 90, disposed beyond
device 88, provides an additional 45.degree. rotation to each
component before reflection and again after reflection. The
reflected beams are now horizontally polarized and are shifted over
to the next port position as they re-traverse device 88.
In accordance with the present invention, the signal components
thereafter encounter a partially transmissive reflector 92. As with
the other embodiments described above, a majority of the optical
signal will be reflected by device 92, and propagate back the
partial circulator 70 in the reverse direction (such that a signal
originating from signal path 74 will be injected into signal path
76, for example). The transmitted portion of the optical signal
then enters a first optical monitoring arrangement 94 of monitoring
system 72, where first optical monitoring arrangement 94 may
comprise any suitable form discussed above (such as, but not
limited to, a large area photodiode, lensed diode, optical fiber,
etc.). In a similar manner, a signal entering second port 76 will
pass through partial circulator 70 and a transmissive portion will
be coupled into a second optical monitoring arrangement 96 and,
similarly, a signal coupled into third port 78 will be monitored by
a third optical monitoring arrangement 98.
FIG. 8 illustrates an optical system 100, including a Faraday
reflector, with a detector arrangement 110 of the present
invention. As shown, an optical fiber 102 is used to provide the
input optical signal. The optical signal thereafter propagates
through a focusing means 104, such as collimating lens, and a
Faraday rotator 106. Faraday rotator 106 provides a 45.degree.
clockwise rotation on each polarization component passing through.
A partially transmissive device 108 is disposed beyond Faraday
rotator 106 and functions to allow only a relatively small portion
of the optical signal to pass through to detector arrangement 110.
The remainder of the optical signal will be reflected, rotated
another 45.degree. by Faraday rotator 106 and be re-injected into
path (or port) 102. The transmitted portion of the signal entering
monitor 112 is then used to assess the performance of the
system.
It is to be understood that the optical tap monitoring system of
the present invention may be used with any Faraday-based reflective
optical system and, further, various other isolator and circulator
arrangements besides those illustrated may be utilized with the
optical tap monitoring system of the present invention.
* * * * *